Subcribing to events

Emitting events

Events emitted from smart contracts and go plugins. These events can be subscribed to in two ways- via Redis or via web sockets.

#Event structure

The event JSON shown above is wrapped in some transaction specific metadata before being emitted to the event stream. The other fields in the metadata include Called address, the contract address, the contract name, and the raw transaction request data.

Below is an example of the full message that goes into Redis:

{
  "caller": {
    "ChainID": "default",
    "Local": "2DiOmpX+kSRZK0jxEgxdGuPC0eo="
  },
  "address": {
    "ChainID": "default",
    "Local": "4ojW7scVDWoi/eM/CqLYHgZZHE0="
  },
  "plugin": "etherboycore:0.0.1",
  "blockHeight": 65,
  "encodedData": "eyJPd25lciI6ImdvZGJvbGUxIiwiTWV0aG9kIjoic2F2ZXN0YXRlIiwiQWRkciI6IjJEaU9tcFgra1NSWkswanhFZ3hkR3VQQzBlbz0iLCJWYWx1ZSI6MTAxMH0=",
  "rawRequest": "CglTYXZlU3RhdGUSGhIIZ29kYm9sZTEaDnsiVmFsdWUiOjEwMTB9"
}

The rawRequest and the encodedData are base64 encoded with standard encoding.

#Subscribing via Redis

By default, the zeknd-sdk will only emit events to the log. To configure it to send it to a Redis sorted set, add the following line to the zeknd.yaml config file:

EventDispatcherURI: "redis://localhost:6379"

This will start emitting events to the Redis server in a sorted set called zekndevents. Each event is added to the sorted set with the score being the blockchain height.

#Subscribing via web sockets

The zeknd SDK query endpoint can be used to subscribe to the event stream as well. Here is a sample subscription code using a command line and the wscat nodejs cli.

$ cat command.json
{
    "method": "subevents",
    "jsonrpc": "2.0",
    "params": [],
    "id": "dontcare"
}

$ wscat -k ws://localhost:46658/queryws < command.json
{
  "jsonrpc": "2.0",
  "id": "dontcare",
  "result": {}
}{
  "jsonrpc": "2.0",
  "id": "0",
  "result": {
    "caller": {
      "ChainID": "default",
      "Local": "2DiOmpX+kSRZK0jxEgxdGuPC0eo="
    },
    "address": {
      "ChainID": "default",
      "Local": "4ojW7scVDWoi/eM/CqLYHgZZHE0="
    },
    "plugin": "etherboycore:0.0.1",
    "blockHeight": 65,
    "encodedData": "eyJPd25lciI6ImdvZGJvbGUxIiwiTWV0aG9kIjoic2F2ZXN0YXRlIiwiQWRkciI6IjJEaU9tcFgra1NSWkswanhFZ3hkR3VQQzBlbz0iLCJWYWx1ZSI6MTAxMH0=",
    "rawRequest": "CglTYXZlU3RhdGUSGhIIZ29kYm9sZTEaDnsiVmFsdWUiOjEwMTB9"
  }
}

The first json in the cli output is the response of the subscribe command. The following json is an event from a contract processing a transaction.

#Example go code

You can refer to a demo indexer which uses the Redis and web socket event subscriptions to read the events and persist to elasticsearch. (Note that this code is meant only for demonstration purposes).